How to make the arms of a rigged character slide on the surface of the body
For Cycles, a trick would be to use a distinct mesh for the arm and use the Bevel node. It will create the illusion of bevel where the meshes are joining, so you'll be able to move the arms however you want and they will still be looking like they are part of the body. Keep the Radius value low (here 0.1).


In Eevee (or Cycles) you could try with a large topology around the arm joint so that it has latitude to move. Give it a Shrinkwrap modifier with a sphere as Target so that it stays spherical:


Or you could use the arm as a distinct object and try the Shrinkwrap modifier, with the Wrap Method &gt; Surface and with a group assigned where the arm is supposed to stick, but it looks hard to get good result: